The most common method of applying viscous medium, such as solder paste, to a substrate, such as a PCB, in electronics production is by using screen printing. The vast majority of viscous medium deposits can be applied by this method. However, in specific circumstances there is a need for different viscous mediums at different locations on the substrate. This poses a problem, since there are a number of difficulties to overcome when adding viscous medium by performing an extra screen printing procedure, after an initial application of viscous medium. Further, it is sometimes desired to produce patterns of viscous medium that are very difficult to achieve through screen printing, such as the pattern of an unfilled circle.
It can also be desired that the applied viscous medium presents different heights at different positions on the substrate in order to accommodate, e.g., chip components having different heights, components having different lead angle accuracy, or co-planarity requirements. A solution to the latter problem that has been suggested in the art is to use so called stepped stencils for the screen printing. However, these stencils present a number of drawbacks that have prevented widespread use of this solution.
Also, the application of very small deposits, primarily circular, i.e. dots, of viscous medium is not possible to obtain through screen printing, since the adhesion forces from the stencil would become to great in comparison to the adhesion forces from the substrate. As a consequence, very small deposits would attach to and follow the stencil when the stencil is removed. It could be imagined that this problem is overcome by using a thinner stencil. However, that would have undesired drawbacks in that it would result in an overall decrease of the height of the deposits, and thus the overall volume, which is undesired and would pose a problem for larger deposits.
According to the state of the art, efforts have been made to solve the above stated problems by using a conventional dispensing equipment. By using a conventional dispenser it is possible to add viscous medium following an initial screen printing of the substrate for producing said patterns of viscous medium that are not easily achieved through screen printing. With a conventional dispenser it can also be possible to apply viscous medium having a different height at different locations on the board. However, the conventional dispensers have a number of disadvantages. The process of conventional dispensing of viscous medium is relatively slow, the application of very small deposits of viscous medium through conventional dispensing is difficult, and it is also difficult to apply additional viscous medium onto a location on the substrate that has already been provided with viscous medium.